The present invention relates to a touch screen display and, more particularly, to a reflection resistant, touch sensitive display.
A touch screen display permits a user to input information to a computer system by touching an icon or other visual element displayed on a screen or by tracing a symbol on a screen to be identified and interpreted by the computer system. Direct user interaction through a touch sensitive screen is considered to be one of the most intuitive methods of computer input. As a result, touch screens have been widely applied to personal digital equipment; to public access data processing systems, such as self-service fuel pumps, automated teller machines and automated ticketing systems; and to instrumentation and controls for medical equipment, aircraft, and vehicles.
Several types of touch screens have developed to address the needs of the wide variety of potential applications. Surface Acoustic Wave (SAW) touch screens comprise a glass panel with acoustic transceivers attached to three corners and reflecting stripes arranged along the edges. The transceivers generate inaudible sound waves that travel across the screen. When a user's finger makes contact with the screen, a portion of the wave's energy is absorbed. The touch screen controller detects the energy loss and calculates the coordinates of the contact. A Near Field Imaging (NFI) touch screen comprises a base layer and a front layer of glass separated by a transparent conductive film deposited in a patterned topology. A controller applies an excitation waveform to the conductive layer to generate a low strength electrostatic field in the front layer of glass. The field is modulated when the glass is contacted by a finger or a conductive stylus, producing a differential signal that is detected and spatially resolved by the controller to determine the location of the contact with the screen.
Capacitive touch screens comprise multiple layers of glass with a thin conductive film between a pair of glass layers. A narrow pattern of electrodes is placed between glass layers. The conductive film may be, for example, patterned indium tin oxide (ITO) or a thin wire mesh. An oscillator circuit attached to each corner of the screen induces a low voltage electric field in the coating. When the glass screen is touched, the properties of the electric field change. The touch screen's controller computes the coordinates of the point of contact with the screen by measuring the relative changes of the electric field at a plurality of electrodes.
The most popular type of touch screen is a resistive touch screen. Resistive touch screens comprise a substantially rigid substrate and a flexible cover each having a surface coated with a transparent conductive material, usually indium tin oxide (ITO). The substrate and cover are bonded together with the conductive surfaces facing each other but separated by an air gap produced by a pattern of transparent insulators deployed on one of the surfaces. When a user presses on the flexible cover, the cover is deformed and the conductive surfaces make contact. A controller measures the voltage drop in circuits resulting from contact between the conductive layers to determine the coordinates of the point at which the contact was made.
Resistive and capacitive touch sensitive systems are typically produced as a transparent, touch-sensitive panel that is placed in front of the screen or display surface of the underlying electronic display. The touch sensitive systems are commonly used in conjunction with several types of displays including cathode ray tubes (CRTs) and liquid crystal displays (LCDs). LCD-based displays are preferred for many touch screen applications because LCDs are lighter, more compact, more rugged, and use less energy than CRT displays.
Generally, an LCD comprises a light valve that controls the intensity of light passing through the panel from a source at the back of the LCD (a “backlight”) to a viewer's eyes at the front of the panel. The light valve generally comprises a pair of polarizers separated by layer of liquid crystals filling a cell gap between the polarizers. The optical axes of the two polarizers are arranged relative to each other so that light from the backlight is either blocked or transmitted through the polarizers. The liquid crystals are birefringent and translucent and the relative orientation of the crystals of the layer can be controlled to switch the light valve from a transmitting state where light is transmitted through the two polarizers to a non-transmitting state where light transmission is blocked. For example, the walls of the cell gap may be buffed to create microscopic grooves that orient adjacent molecules of liquid crystal with optical axes of the two polarizers. Liquid crystals exhibit a dipole that attracts neighboring crystals and causing the crystals of columns spanning the liquid crystal layer to align with each other. If the crystals at the limits of the layer are arranged at an angle to each other to align with the optical axis of the polarizers, the crystals of the intervening column will be progressively twisted into alignment by the dipole. The plane of vibration of light transmitted from the first polarizer passing through a column of crystals is also “twisted” so that it is aligned with the optical axis of the second polarizer and visible to the viewer (in a “normally white” LCD). To turn a pixel off and create an image, a voltage is applied to an electrode of an array of electrodes on the walls of the cell gap with reference to a common electrode causing adjacent liquid crystals to be twisted out of alignment with the optical axis of the adjacent polarizer attenuating the light transmitted from the backlight to the viewer. (Conversely, the polarizers of the light valve of a “normally black” LCD are arranged so that the pixel is “off” or “black” when the controlling electrode is not energized and switched “on” or “white” when the electrode is energized.)
While LCDs are the displays of choice for many touch screen applications, the combination of an LCD display device and a touch panel can be problematic. The principal problem is that touch panels are reflective and, when exposed to intense ambient lighting, the luminance of the reflection often overpowers the image being displayed by the LCD.
The reflectivity of a resistive touch panel is principally the result of coating the facing surfaces of the cover and the substrate with the transparent conductive coating of ITO. ITO has a relatively high index of refraction (typically, n=1.83 for light in the green wavelengths) while the air in the gap between the resistive surfaces has an index of refraction of 1.0. The percentage of perpendicularly incident light reflected from a discontinuity in the index of refraction is, approximately:
      R    ⁡          (      %      )        =            (                                    n            2                    -                      n            1                                                n            2                    +                      n            1                              )        2                  where:                    n2=index of refraction for the optically denser material            n1=index of refraction for the optically less dense material            R=percentage of incident light reflected(It is noted that the aforementioned equation is only accurate for perpendicular viewing directions.) The result of two transitions of the air-to-ITO boundaries by ambient light from the front of the panel is reflection of roughly 17% of the ambient light incident on the panel. This reflection is sufficient to obscure the displayed image under modest to high intensity ambient lighting conditions.                        
Sawai et al., U.S. Pat. No. 6,020,945, disclose an optical filter for a resistive touch panel that is intended to prevent reflection of external light and obscuration of the displayed image. The optical filter comprises, generally, a filter polarizer in front of the screen of the display. The filter polarizer may be a circular polarizer comprising a combination of a linear polarizer and a quarter wave phase difference plate. Much of the ambient light striking the front of the panel is absorbed by the filter polarizer. In addition, light passing through the filter polarizer and reflecting from the ITO layers passes twice through a quarter wave phase difference plate. The phase difference plate alters the polarization of the reflected light so that the reflected light is blocked by the filter polarizer.
On the other hand, the linear polarized light of the image from the LCD light valve is circular polarized by a second phase difference plate before passing through the touch panel. The optical axis of a circular polarizing, filter polarizer is aligned so that the circular polarized light from the LCD is transmitted through the filter polarizer to the viewer. While a touch panel filter reduces reflection of ambient light, the combination of the filter polarizer and phase difference plates substantially attenuates the light from the light valve reducing the brightness of the image, and the combination of the touch panel and filter substantially distort the image.
What is desired, therefore, is a touch screen providing substantially reduced reflection of ambient light and an undistorted image.
TABLE 1A is a table of alternate reflection resistant touch screen constructions indicating arrangements and characteristics of touch screen elements.
TABLE 1B is a table of the specular reflection contributions from the components of the touch screens of alternate construction described in TABLE 1A.
TABLE 1C is a table of the diffuse reflection contributions from the components of the touch screens of alternate construction described in TABLE 1A.
TABLE 1D is a table illustrating the optical performance of the touch screens of alternate construction described in TABLE 1A.